Method of coating



United States 2,858,235 Patented Oct. 28, 1958 tice METHOD OF COATINGCarl A. M. Rex, Hohoken, N. J., assiguor of one-half to Jack F. Govanand one-half to Roy H. Gavan, both of Hoboken, N. J.

No Drawing. Application March 17, 1953 Serial No. 342,985

9 Claims. (Cl. 117 455) The present invention relates to a process forthe coating of metals and other materials and the resulting products,and more particularly to a process for the coating of metal surfaces orthe like to protect such surfaces when exposed to heat, oxidizingconditions, corrosive vapors, mechanical erosion, and the like.

Prior to the instant invention, many processes and coating materialshave been proposed for use in protecting metallic surfaces. However, ithas been found that the articles containing these coatings have beenunsatisfactory in one or more of the following respects. At hightemperatures, the coatings have been found to decompose and burn andthus lose all of their protective characteristics. The enamel andceramic glazes, due to the methods of application used, have been foundincapable of withstanding severe thermal and percussive shock. Suchcoatings have been found to be incapable of adhering well to the basemetal during usage. None of the coatings nor processes of coatingpreviously proposed has been found capable of being applied to metalsand other surfaces of material having widely different expansioncoefiicients such as iron, steel, aluminum, copper, stainless steel,tungsten, molybdenum, magnesium, titanium, and the like, and which willadhere to such a variety of surfaces when thermal shock is imparted tothe coating, such as by heating followed by sudden cooling. In suchcases, the differences in the expansion of the metals and the coatingoccurs so suddenly as to set up enormous stresses and strains whichdislodge the coatings. Further, the coatings as previously applied willnot-withstand flexing, bending, forming, swaging, or drawing of thecoated surface without dislodging the coating.

An object of the present invention is to provide a method of coatingmetals and other surfaces to yield a protecfively coated surface thatwill withstand flexing, bending, forming, swaging or drawing withoutdislodging the protective coating so applied.

A further object of the present invention is to provide a method ofprotectively coating metals and the like to yield a coating capable ofprotecting such materials during heat treatment.

A still further object of the present invention is to provide a methodof protectively coating metals and the like whereby not only the surfaceof the material is coated, but wherein the coating extends to a slightdepth beneath the surface, however, the core of the body of material isnot affected.

A still further object of the present invention is to provide a methodof protectively coating metals and the like whereby the resultantproduct is capable of withstanding thermal and percussive shockregardless of the diflerences in coeflicients of the coating and themetals coated thereby.

Other objects and the nature and advantages of the instant inventionwill be apparent from the following description, wherein through the useof powder metallurgy techniques or the equivalent a composite materialis to be entirely successful.

formed which consists of a high density core and an intimately bondedprotective coating of a vitreous or ceramic material which extends to aslight depth beneath the surface of the material. In general, theprocess includes three broad steps, namely, the formation of a poroussurface, the coating thereof with a specific type coating, and thedensification of the coated body.

The metal to be protected may be obtained in the form of finely dividedmetallic powder and placed under suitable pressure to cause the powderto cohere into a porous body of relatively low density. Metal powders ofvarious kinds may be employed in the practice of the invention, such asiron, chromium, nickel, copper, aluminum, silver, tin, zinc, molybdenum,tungsten, magnesium, titanium, and alloys of these metals, such asstainless steel, etc.

The more conventional methods of powder making involve the crushing andgrinding of metal ingots or the like to substantially minute ormicroscopic powder particles, preferably of approximately 450 mesh.Metal powders are also made by reducing the purified oxide of the metalsin a correct atmosphere, such as hydrogen or hydrogen and nitrogen. Thedeposition of pulverulent: metal directly in an electrolytic tank offersan excellent source of powder.

The metal powders may be compressed either as an individual metal, as amixture of two or more metals, or as powdered alloys. The initialcompression is effected at a The compacted briquette or other articlehaving a porous surface is then coated with the protective material byany of the known methods, such as dipping, spraying,

brushing, soaking, Wiping or flooding. The coating mate rial is thenallowed temperatures as desired. Due to the porous nature of the bodybeing coated, the coating material will penetrate to a depth slightlybelow the where the protection is desired such a depth Where it willaffect the core of the metal.

The vitreous or ceramic coating composition to be used is an importantpart of the invention, since not all vitreous or ceramic coatingmaterials have been found A coating composition as described inapplicants copending application Serial No. 205,035, filed January 8,1951, entitled Method of Protecting Metal Surfaces, CompositionTherefor, and Article Resulting Therefrom, now Patent 2,658,834, hasbeen found particularly useful. This coating composition includes sodiummeta borate, preferably in combination with clay and a reducing agent.

Actually, sodium meta borate composition produces a vitreous ofprotection which, for some When heated, it forms a kind of alone as thecoating coating with a degree purposes, is desirable. glass or vitreouscoating upon mere drying results in a coating that will in the outerpores and surface of the porous mass which acts to protect the metalsurface to a fairly high temperature, as during the period of heatingprior to densification of the mass. While a coating of sodium metaborate,

when applied in accordance with the present invention,

also gives a degree of protection where the part is to be placed in hightemperature service for prolonged periods of time, the incorporation ofclays, reducing agents, and other pigments with the meta borate to yielda ceramic coating improve the higher temperature protectionconsiderably.

In the claims, it should be understood that the terms fvitreous coatingand vitreous coating composition are meant to include the use of sodiummeta borate alone as the coating composition, the combination of sodiummeta borate with other additives to yield a glass like coating, as wellas the combination of sodium meta borate with clay and other additivesto yield a ceramic type coating.

The sodium meta borate utilized alone or in the coating composition canbe any of the materials commonly known as sodium meta borate, hydratedor anhydrous. Following are several examples which have been found towork successfully:

NaBO -2H O NaBO -4H O Instead of adding the sodium meta borate, in theabove forms, equally good results can be obtained by admixing Na O and Bin mol ratios substantially equal to that existing in sodium metaborate. The sodium and boron compounds added can consist of alkalies ofsodium together with oxides or acids of boron which, in combination,will give a sodium meta borate. Examples of the sodium alkali suitablefor use are sodium oxide, carbonate, bicarbonate, hydroxide, or anyorganic sodium salt which upon thermal decomposition or ignition willgive an alkaline ash of sodium oxide or carbonate. The boron compoundused can be boron oxide, boric acid, borax or the like.

The borate acts as a flux for the clay, when used in combination andheated to form a ceramic coating. A suitable clay for use in the coatingis a plastic clay. Such a clay, when mixed with water, assumes a plasticconsistency and includes a rather narrow range ofclays foremost of whichare the ball clays of the ceramic industry. Clays which are notpossessed of this plasticity are not suitable for use in the presentinvention.

The reducing agent, or oxidation inhibitor, when elected forincorporation in the coating, must be comminuted to small particle sizeso as to distribute itself smoothly throughout the protective layer, toreduce the tendency to separate and stratify while the coating is airdrying, and subsequently to present a maximum surface to contact thecorrosive gases and vapors that contact the coating. Suitable reducingagents or oxidation inhibitors include graphite, carbon black, andpowdered metals, alloys or non-metals, such as iron, chromite (FeO-Cr Ocopper, silicon, and the like. Other oxides, pigments or the like may beincorporated, such as titanium dioxide, even if they are neitheroxidizing or reducing in nature.

In the application of the material to the surface to be coated, it ispreferable that water he used to eifect solution of the solubleingredients and suspension of the insoluble ingredients, usingsufficient liquid to form a slurry having the consistency of paint. If,in addition, a proportion of an organic adhesive or binding agent isadded thereto either with the original dry materials or with the waterused for solution and dispersion a better adhesion of the wet coating tothe work is secured, and withstand more handling and abuse than oneprepared without the benefit of such adhesive material. Therefore, whenapplying the coating to the work it is deemed preferable to include anadhesive agent, such as dextrine, and the preferred embodiment of theinvention makes use of the same. It is, of course, understood that anysimilar adhesive such as starch, gum, gum acacia, gum tragacanth, etc.can be utilized in place of the dextrine quite as effectively.

The inclusion of such adhesive is for the primary purpose of imprintingdesirable properties to the coating before heating, for upon heating theadhesive is decomposed at a relatively low temperature. However, by thistime the natural adhesiveness of the coating itself begins to come intoplay and, of course, at high temperature the adhesiveness of the fusedmass is brought into full force and effect.

In the application of this invention it has been found that thefollowing range of proportions is satisfactory:

Organic binder The following specific example is intended to illustratethe preparation of a suitable coating for use in connection with thepresent invention, but is not intended to limit same:

50 lbs. powdered graphite 20 lbs. dextrine lbs. Kentucky ball clay aremixed in a paddle mixer until all of the ingredients are uniformlydistributed.

In a separate mixer, lbs. sodium meta borate octa hydrate (Na B O -8H O)and 25 gallons water are mixed until the borate is dissolved using heatif necessary.

To this solution, the dry ingredients are added while agitating, alittle at a time. When all have been added the mixture is mixed for twoand a half hours until all ingredients have been thoroughly andintimately distributed. In this form the batch is ready for use.

The coated briquette or body is heated to a temperature approximatingthe normal forging temperature for the metal in question. The heatingmay be accomplished in a furnace or by passing a suitable current ofelectricity through the mass. The time and temperature of heating arecarefully regulated for the particular metal and coating, after whichthe heated and coated briquette or body is densified.

The densification can be accomplished by mechanical working such as byforging, rolling or hot pressing. A pressure of approximately 300 to 500lbs. per square inch has been found to yield good results. When forgingone to three sharp blows at this pressure and temperature has been foundto yield the desired densification.

During the densification, a certain amount of the coating is forced bythe heat and pressure to penetrate be neath the outer surface of themetal, thus establishing a mechanical bond between the metal and thecoating, and it is further believed that a partial chemical orphysicochemical bond of the metal and the coating material is formed inthis area, thus increasing the bonding tendency and introducing an areaof transition from pure metal on one side to coating material. on theother.

It is contemplated that the coating material used will 'cated into rods,wires,

be capable of being fused under the influence of the pressure and heatapplied during the heating and densifying steps, and upon cooling willsolidify in vitreous form and assume a metallic luster. At the elevatedtemperture, the coating material is soft and plastic and whensubsequently struck with the great force necessary to density the metal,it is brought into intimate contact with and is worked into the metal toa slight depth. Thus, there results an outer continuous coating over apartially continuous phase of both vitreous composition and metalconfined to the area adjacent to the surface to a slight depth whichispermanently held for the protection of the metal against any attack towhich it is normally subject, but to which ceramics and vitreousmaterials are known to be resistant.

In the subsequent working of the metal, coated in accordance with thisinvention, such as by forming, bending, drawing, swaging or the like,this protective coating remains intact and continues to exert isprotective function. In-this manner, the present invention affords amethod of protecting the metal while it is fabrisheets or finishedarticles consisting of a core of a completely dense metal, an exteriorshell made up of a combination of metal and vitreous or ceramicmaterial, and an outer coating of vitreous or ceramic material.

' In certain cases, the steps of coating and densifying as describedheretofore may be repeated several times as necessary to obtain thedesired results.

An advantage of this process of coating is that the metal article can besubsequently worked to change its physical dimensions without therequirement for a protective atmosphere or a subsequent recoating.

It is further contemplated that the porosity required on the surface ofthe metal to be coated can be produced by spraying the surface withmolten metal. The remaining steps in the process would be identical tothose heretofore described.

The process described herein lends itself particularly well to theformation of coated filaments of tungsten or molybdenum wherein a coatedrod is formed from the powdered metal in accordance with the presentinvention and after densification, the rod can be hot drawn into afilament which retains the protective coating held by the coated rod.Such coated articles of molybdenum, for example, have been foundsuitable for use in high temperature service, whereas previouslymolybdenum has been found unsatisfactory due to its great tendency tooxidize at high temperatures.

The following is a specific example of the process in accordance withthe present invention, but is not intended to limit the same:

Molybdenum in powdered form was compressed into a briquette by coldpressing at 12%. tons. The resulting briquette was coated by brushingwith a coating material having the following composition:

95 lbs. sodium meta borate octa hydrate (Na B O 8H O) 75 lbs. Kentuckyball clay 20 lbs. dextrine 10 lbs. powdered graphite 26 lbs. titaniumdioxide 25 gals. water After air-drying the coating, densified byheating to 2300 F. for fifteen minutes and a pressure of 200 lbs. persquare inch was applied. The densified briquette was recoated, dried andfurther densified by heating to 2100 F. for ten minutes and applying apressure of 200 lbs. per square inch. The resulting bar upon sectioningand polishing showed that the coating material had penetrated less thanone quarter of the distance from the outer surface of the bar to thecenter thereof.

The field of application of this coating process is extremely varied anddiverse and includes such applications the coated briquette was 1changes may be made without departing from the spirit of the inventionand, therefore, the invention is not limited to what is described in thespecification, but only as indicated in the appended claims.

What is claimed is:

1. A method of protecting metal articles from oxidation at hightemperatures which metal articles are in the form of a porous briquettemade by compacting finely divided metal powder which comprises applyinga liquid vitreous coating composition to the surface of said briquette,said coating composition containing sodium meta borate as an ingredienttherein, drying said coating, and densifying said briquette under heatand pressure, said densifying being accomplished by hot forging saidcoated porous article at a pressure of at least 200 lbs. per square inchwhile the temperature of the article is approximately at the normalforging temperature of said metal, whereby the base metal of saidarticle is densified and at the same time said coating composition haspenetrated a slight depth below the outer surface of said metal and isvitrified thereon.

2. A method in accordance with claim 1, wherein the product resultingtherefrom is again coated with said vitreous coating, said coating isdried, and said coated product is again densified.

3. A protectively coated metal article obtained in accordance with themethod of claim 1.

4. A protectively coated metal article obtained in accordance with themethod of claim 2.

5. A method of protecting a porous molybdenum article against oxidationat high temperatures which article has been formed by compacting finelydivided molybdenum powder into said porous article, applying a liquidvitreous coating composition to the surface of said article, saidcoating composition containing sodium meta borate as an ingredienttherein, drying said coating, and densifying said article under heat andpressure, said densifying being accomplished by hot forging said coatedporous article at a pressure of at least 200 lbs. per square inch whilethe temperature of the article is approximately 2300 F., whereby thebase metal of said article is densified and at the same time saidcoating composition has penetrated a slight depth below the outersurface of said metal and is vitrified thereon.

6. The method in accordance with claim 5, wherein said article is hotdrawn into a wire, said wire still retaining the densified core ofmolybdenum and the coating of said vitreous material.

7. The method in accordance with claim 5, wherein said articleis againcoated with said vitreous coating composition and densified under heatand pressure.

8. A protectively coated metal article obtained in accordance with themethod of claim 5.

9. A protectively coated metal article obtained in accordance with themethod of claim 7.

References Cited in the file of this patent UNITED STATES PATENTS1,365,499 Kelley Jan. 11, 1921 2,102,539 Lauenstein et al Dec. 14, 19372,294,760 Morris Sept. 1, 1942 2,566,752 Stern Sept. 4, 1951 2,581,252Goetzel et al Jan. 1, 1952 2,658,834 Rex Nov. 10, 1953 2,709,516Trembicki May 31, 1955 2,755,199 Rossheim July 17, 1956 2,826,512 RexMar. 11, 1958

1. A METHOD OF PROTECTING METAL ARTICLES FROM OXIDATION AT HIGHTEMPERATURES WHICH METAL ARTICLES ARE IN THE FORM OF A POROUS BRIQUETTEMADE BY COMPACTING FINELY DIVIDED METAL POWDER WHICH COMPRISES APPLYINGA LIQUID VITEROUS COATING COMPOSITION TO THE SURFACE OF SAID BRIQUETTE,SAID COATING COMPOSITION CONTAINING SODIUM META BORATE AS AN INGREDIENTTHEREIN, DRYING SAID COATING AND DENSIFYING SAID BRIQUETTE UNDER HEATAND PRESSURE AND DENSIFYING BEING ACCOMPLISHED BY HOT NOT FORGOING SAIDCOATED POROUS ARTICLE AT A PRESSURE OF AT LEAST 200 LBS. PER SQUARE INCHWHILE THE TEMPERATURE OF THE ARTICLE IS APPROXIMATELY AT THE NORMALFORGOING TEMPERATURE OF SAID METAL, WHEREBY THE BASE METAL OF SAIDARTICLE IS DENSIFIED AND AT THE SAME TIME SAID COATING COMPOSITION HASPENETRATED A SLIGHT DEPTH BELOW THE OUTER SURFACE OF SAID METAL AND ISVITRIFFIED THEREON.